Thermodynamics is a branch of physics
which deals with the energy and work of a system.
Thermodynamics deals only with the
large scale response
of a system which we can observe
and measure in experiments. Small scale gas interactions are
described by the kinetic theory of gases.
There are three principal
laws of thermodynamics which are described on separate slides. Each
law leads to the definition of thermodynamic properties
which help us to understand and predict the operation of a physical
system. We will present some simple examples of these laws and
properties for a variety of physical systems, although
we are most interested in the thermodynamics of
propulsion systems
and
high speed flows.
Fortunately, many of the
classical examples involve gas dynamics. Unfortunately, the numbering
system for the three laws of thermodynamics is a bit confusing. We
begin with the zeroth law.

The zeroth law of thermodynamics begins with a simple definition
of thermodynamic equilibrium . It is observed that some
property of an object, like the pressure in a volume of gas, the
length of a metal rod, or the electrical conductivity of a wire, can
change when the object is heated or cooled. If two of these objects
are brought into physical contact there is initially a change in the
property of both objects. But, eventually, the change in property
stops and the objects are said to be in thermal, or thermodynamic,
equilibrium. Thermodynamic equilibrium leads to the large scale
definition of temperature. When two objects
are in thermal equilibrium they are said to have the same
temperature. During the process of reaching thermal equilibrium,
heat, which is a form of energy, is transferred
between the objects. The details of the process of reaching thermal
equilibrium are described in the
first and
second
laws of thermodynamics.

The zeroth law of thermodynamics is an observation. When two objects are
separately in thermodynamic equilibrium with a third object, they are in
equilibrium with each other.
As an illustration, suppose we have three objects as shown on
the slide. Object #1 and object #2 are in physical contact and in
thermal equilibrium. Object #2 is also in thermal equilibrium with
object #3. There is initially no physical contact between object #1
and object #3. But, if object #1 and object #3 are brought into
contact, it is observed that they are in thermal equilibrium. This
simple observation allows us to create a thermometer. We can
calibrate the change in a thermal property, such as the length of a column of
mercury, by putting the thermometer in thermal equilibrium with a
known physical system at several reference points.
Celsius thermometers have the reference
points fixed at the freezing and boiling point of pure water.
If we then bring the thermometer into thermal equilibrium with any other system,
such as the bottom of your tongue, we can determine the temperature of the other
system by noting the change in the thermal property.
Objects in thermodynamic equilibrium have the same temperature.